In the machining of metal parts, there are configurations and operations which become quite difficult and result in less than ideal pieces, with some inadequate parts that must be thrown away, or with product lines that do not maximize desired operating characteristics as a consequence of the inability to improve the manufacturing processes. An example is hollow shafts used in modern gas turbine engines, such as those typically used on aircraft. In hollow engine shafts, it is common to have internal diameters along the central portions of the shaft which are larger than the internal diameters of the shaft near its ends. This means that the shafts cannot be produced by simply drilling the internal voids, but rather require more complex machining processes. In such process, a boring machine marketed under the tradename Alfing, utilizes solidified sulfur as a guide to a boring bar which is drawn through the area to be machined, thus "hogging" the material out of the part (the process is itself sometimes referred to as Alfing). This process is not very accurate and leaves internal surfaces which are not perfectly round and not perfectly centered along a straight line. On the other hand, the external surface of such a shaft must be perfectly centered, straight, round and smooth (within a few mils) in order to provide proper mating with the various other engine parts, and acceptable relative motion therewith.
The level of vibration in the operation of an engine resulting from uneven wall thicknesses at various points along the shaft can only be partially reduced through compensation. The only way to reduce gas turbine engine vibration as a consequence of its main shaft imbalance is to improve the machining process and thereby reduce the vibration which would occur prior to compensation therefor.
It is known to use the timing of ultrasonic echoes to determine characteristics and dimensions of workpieces being machined. Using ultrasound to measure surfaces of parts being machined is shown in U.S. Pat. No. 4,655,084. It is also known to communicate the ultrasonic pulses through fluids involved in machining processes. U.S. Pat. No. 4,272,924 discloses measuring the thickness between the upper and lower surfaces of glass workpieces which are being lapped with a lapping fluid, the ultrasonic wave communicating to the workpiece through the lapping fluid. Coupling the acoustic waves to the workpiece through a fluid is also shown in U.S. Pat. No. 4,976,149. However, none of these patents address, even remotely, any part of the foregoing problem.